1. Field of the Invention
The present invention relates to an organic electroluminescent device.
2. Description of the Related Art
An old-style organic electroluminescent device has been caused to emit light by, for example, applying a voltage to an anthracene deposited film (Thin Solid Films, 94, 171 (1982)). Since, for example, the fact that an organic electroluminescent device can be driven at a low voltage and the fact that the device is superior in response speed to a liquid crystal display (LCD) have been revealed in recent years, the device has been expected to provide practical use in, for example, displays, and the development of a material for the device with a view to putting the device into practical use has been vigorously conducted.
Here, as detailed in Macromol. Symp. 125, 1 to 48 (1997), the structure of the organic electroluminescent device is generally such that two electrodes, that is, upper and lower electrodes and layers each formed of an organic compound, the layers being interposed between the electrodes and the layers including a light-emitting layer, are formed on a transparent substrate.
By the way, a technology for causing the organic electroluminescent device to emit light conventionally adopted is as follows: fluorescence upon transition from a singlet exciton to a ground state is utilized as a light emission output. Not only fluorescence but also phosphorescence upon transition from a triplet exciton to a ground state as disclosed in each of Improved energy transfer in electrophosphorescent device (D. F. O'Brien et al., Applied Physics Letters Vol. 74, No. 3, p. 422 (1999)) and Very high-efficiency green organic light-emitting devices based on electrophosphorescence (M. A. Baldo et al., Applied Physics Letters Vol. 75, No. 1, p. 4 (1999)) has been recently utilized as a light emission output. In addition, each of Improved energy transfer in electrophosphorescent device (D. F. O'Brien et al., Applied Physics Letters Vol. 74, No. 3, p. 422 (1999)) and Very high-efficiency green organic light-emitting devices based on electrophosphorescence (M. A. Baldo et al., Applied Physics Letters Vol. 75, No. 1, p. 4 (1999)) proposes an organic electroluminescent device having a four-layered constitution (formed of a hole-transporting layer, a light-emitting layer, an exciton diffusion-prevention layer, and an electron-transporting layer). An improvement in the efficiency of the organic electroluminescent device is achieved by using a green phosphorescent material Ir(ppy)3 as a light-emitting material contained in the light-emitting layer.
Meanwhile, in addition to such organic electroluminescent device using a low-molecular-weight material as described above, an organic electroluminescent device using a conjugated polymer has been reported by a group in the University of Cambridge (Nature, 347, 539 (1990)). The report proposes a device that emits light by using a single layer formed of polyphenylene vinylene (PPV) by an application method (Wet process). Here, the advantages of the production of an organic electroluminescent device by the application method are, for example, as follows: the application method is a simple method for film formation as compared to, for example, a deposition method, and eliminates the needs for a shadow mask for separately coating coloring materials and a vacuum apparatus. The application method is a promising method of producing an organic electroluminescent device because of those advantages.
As described above, a technique for producing an organic electroluminescent device has recently shown significant progress. However, the device produced by the technology still requires an optical output with a higher luminance or higher conversion efficiency in order that the device may be put into practical use, so that a technology for producing a device that brings together high durability and high color purity has been demanded. In particular, as far as organic electroluminescent devices for emitting green light or blue light are concerned, the reality is as follows: the number of devices at practical levels each of which brings together a high efficiency, high durability, and a high color purity has been still small. In addition, as far as only a device corresponding to the application method is concerned, it is difficult even to procure a material for producing such device at a practical level.
A blue phosphorescent device having a high efficiency has been recently proposed in Applied Physics Letters, 83, 3, 818 (2003). Applied Physics Letters, 83, 3, 818 (2003) discloses, as a host material to be used in a light-emitting layer, a Si-containing compound formed of UGH1 and UGH2. However, the Si-containing compound disclosed in the document is limited to one having a simple structure substituted with a phenyl group, and the material itself has high crystallinity. Accordingly, the use of the compound as a component for an organic electroluminescent device is apt to cause the crystallization at the time of the driving of the device or due to the long-term storage. In addition, the Si-containing compound shows so poor solubility in any one of various solvents that it is difficult to produce a device from the compound by employing the application method.
As described above, as long as attention is paid only to an improvement in the efficiency, several organic electroluminescent devices at practical levels have been reported, but a hurdle which those who wish to obtain an organic electroluminescent device that brings together high durability and a high color purity as well as a high efficiency must overcome is high.